The　scarcity　of　rapidly　biodegradable　organics,　which　serve　as　essential　electron　donors　for　the　partial　denitrification　(PD)　process,　significantly　hinders　the　combined　application　of　PD　coupled　with　anammox　(PDA)　in　municipal　wastewater　treatment　plants.　This　study　innovatively　applied,　for　the　first　time,　a　step-feed　strategy　combined　with　the　use　of　sludge　fermentation　liquid　(SFL)　as　an　external　carbon　source　in　an　integrated　fixed-film　activated　sludge　(IFAS)　system,　successfully　driving　full　nitrification　and　PDA　to　achieve　advanced　nitrogen　removal　from　low　C/N　real　municipal　wastewater.　Moreover,　the　associated　nitrogen　removal　mechanism　of　this　system　was　systematically　analyzed.　By　employing　second-step　SFL　feed　as　a　supplementary　carbon　source,　the　nitrogen　removal　efficiency　reached　92.26　±　2.77　%　and　the　effluent　total　inorganic　nitrogen　was　6.43　±　2.23　mg/L,　with　anammox　contributing　approximately　70　%　to　total　inorganic　nitrogen　removal.　16S　rRNA　gene　sequencing　and　fluorescence　in　situ　hybridization　analysis　unveiled　the　extensive　cooperation　and　synergistic　interactions　among　anammox　bacteria,　denitrifying　bacteria,　and　nitrifying　bacteria,　with　Candidatus　Brocadia　being　highly　enriched　in　biofilms　with　a　relative　abundance　of　2.21　%.　Metagenomic　sequencing　confirmed　that　the　relative　abundance　of　the　narGHI　gene　was　greater　than　that　of　the　nirS　gene,　providing　stable　nitrite　accumulation　conditions　for　the　anammox　process.　Overall,　this　study　proposes　an　innovative　synergistic　treatment　scheme　that　utilizes　a　step-feed　full　nitrification-PDA　process　driven　by　SFL　to　achieve　advanced　nitrogen　removal　in　municipal　wastewater　treatment　plants.　This　approach　is　characterized　by　low　energy　consumption,　low　operational　costs　and　a　high　nitrogen　removal　efficiency.　©　2025　Elsevier　Ltd